Abstract

Amaranthus tuberculatus (Common Waterhemp) is a prevalent, problematic weed in Midwestern USA, where genetically-modified crops are widely grown, resulting in multiple annual applications of glyphosate. Such practices provide the selection pressure for the evolution of glyphosate resistance. Evolutionary theory predicts that adaptation to novel stresses, such as herbicide application, will incur a cost in the original (herbicide-free) environment. This project aims to identify whether glyphosate resistance in a population of Amaranthus tuberculatus was associated with a fitness cost in the absence of glyphosate. Initial dose response experiments on the study population determined a resistance index of 3.2 compared to a standard sensitive population. To generate appropriate material for comparisons between phenotypes, individual plants were cloned and their resistance status determined by glyphosate application. Parent plants were designated as resistant or susceptible and appropriate crosses were performed to generate seed lines. The incomplete segregation of resistant and susceptible seed lines indicated that resistance was controlled by a quantitative trait. EPSPS gene amplification was elucidated as the primary mechanism of glyphosate resistance. There was a strong positive relationship between half-sibling seed family LD50 and relative EPSPS gene copy number indicating that resistance was primarily caused by target-site gene amplification. Subsequent fitness experiments used seed families to determine that no fitness trade-off was associated with resistance (or gene copy number) when plants are grown without competition, in glasshouse and polytunnel growth environments. However, a small growth penalty was associated with elevated gene copy number in plants grown in a polytunnel, without competition (10% biomass reduction at +20 EPSPS copies), this led to competition experiments. A fitness cost was associated with resistance under intra-phenotypic competition; however, the cost was mitigated under inter-specific competition with maize. A cost was associated with glyphosate resistance and this cost was influenced by interactions with biotic and abiotic factors.